The present invention generally relates to the field of bonding conductive and nonconductive bodies together by means of anodic bonding techniques. The present invention more specifically relates to the field of utilizing anodic bonding techniques for manufacturing capacitive pressure sensing elements in which pressure changes alter the spacing between capacitive plates thereby providing changes in capacitance related to sensed pressures.
Capacitive pressure sensing elements are known in which pressure changes result in altering the spacing between capacitor plates so as to provide a change in electrical characteristics indicative of sensed pressure. One such capacitive pressure sensing element is illustrated in U.S. Pat. No. 4,225,632 to Ho which is assigned to the same assignee as the present invention. In such capacitive pressure sensing elements, typically one capacitor plate is mounted on a relatively rigid base substrate while another capacitor plate is spaced apart from the base substrate capacitor plate and is mounted on a flexible diaphragm. In response to pressure changes, the diaphragm will flex thereby changing the spacing between the capacitor plates and providing a change in capacitance representative of the sensed pressure.
Capacitive pressure sensing elements such as those described in the above referred to U.S. Patent provide an internal cavity for storing a fixed or variable predetermined reference pressure within the sensing element such that pressures external to the sensing element diaphragm are measured with respect to this reference pressure. Typically some sort of bonding material is utilized to mount the diaphragm to the base substrate. The use of this bonding material affects the nominal separation between the diaphragm and base substrate capacitor plates and therefore controlling the thickness of this bonding material is critical to insure proper production of the capacitive sensing element. Also the bonding material is used to form part of a hermetic seal for the internal cavity. Such pressure sensing elements are used for sensing automobile engine manifold pressure so as to provide a pressure related capacitance which will be used to electrically control the automobile engine.
Some capacitive sensing elements have eliminated the need for a separate bonding material to bond the diaphragm to the base substrate, thus eliminating the need for controlling the bonding material thickness and for insuring hermeticity of the bonding material itself. This has been accomplished through the use of anodic bonding techniques which bond a thin wafer of conductive semiconductor material, which acts as the pressure sensing diaphragm and one of the capacitor plates, to a relatively thick dielectric glass plate base substrate having a metallization on an exterior surface thereof which functions as the other capacitor plate.
Typically, each individual capacitive sensing element comprises a portion of a semiconductor wafer having a central surface recessed portion and a surrounding nonrecessed portion, and this element functions as the pressure sensing diaphragm. The base substrate of the capacitive pressure sensing element comprises a portion of a glass plate having a conductive metallization centrally positioned on an exterior surface thereof. The diaphragm and base substrate are bonded to each other by anodic bonding techniques such that the recessed portion of the conductive pressure sensing diaphragm is spaced apart and insulated from, but facing, the base substrate metallization with the nonrecessed surrounding portion of the diaphragm being bonded to the base substrate surface on which the base substrate metallization is located. This structure forms a capacitive pressure sensing element having an internal cavity which separates the two electrodes of the pressure sensing capacitor, one of which comprises the base substrate metallization and the other of which comprises the conductive pressure sensing diaphragm. Typically conductive feedthroughs in the base substrate have been utilized to make electrical output connections to the base substrate electrode metallization and the conductive diaphragm.
It is known to anodically bond the conductive semiconductor diaphragm to the glass plate base substrate to manufacture capacitive pressure sensing elements as described above. The present invention involves an improved bonding method for manufacturing such sensing elements. In the known anodic bonding method utilized for construction of these capacitive pressure sensing elements, a negative voltage potential is applied to the glass dielectric plate while a substantial positive voltage potential is applied to the conductive semiconductor diaphragm after the diaphragm and dielectric plate have been placed in contact with each other and properly aligned and after both components have been heated to a substantial temperature. This corresponds to the standard technique of utilizing anodic bonding to provide a bond between conductive and dielectric materials. The use of anodic bonding techniques has therefore eliminated the need for a separate bonding material between the diaphragm and base substrate and has therefore improved the repeatability of manufacturing capacitive sensing elements by providing a substantially constant and predictable value for the nominal capacitance of the pressure sensing element.
While the use of the above-described anodic bonding technique to manufacture capacitive pressure sensing elements is feasible, I have noticed that the use of this prior technique can cause several potential problems. One of these problems is that because of the close spacing typically provided between the base substrate electrode metallization and the conductive diaphragm, a substantial corona field exists between these two elements during the application of voltage potentials during the anodic bonding process. This results in having less energy available to actually form the anodic bond since a substantial amount of energy goes into providing the corona field between the base metallization and the conductive diaphragm. In addition, I noticed that during the anodic bonding process severe arcing can occur in the area between the base electrode metallization and the conductive diaphragm which can result in the vaporization of the base substrate electrode metallization thereby degrading the integrity of this metallization. I also noticed that during the anodic bonding process a substantial bowing of the diaphragm towards the base electrode metallization is likely, and that if the diaphragm touches the electrode metallization this will result in the formation of an undesired silicon gold eutectic. Also, I noticed that when the anodic bonding technique, as described above, was utilized significant solderability problems occurred after the anodic bonding with respect to gold metallizations that were connected to the base substrate electrode and formed the connection points for solder connecting the capacitive pressure sensing element to other electronic components.